The corrosion of metal equipment by petroleum products is a continuing problem in the design, construction and operation of refining equipment. Two principal corrosion mechanisms that can occur in petroleum streams: metal dissolution, for example, by naphthenic acid attack and/or scale formation, for example, sulfide scale formation from reactive sulfur attack. The extent of corrosion which is encountered in actual practice may be controlled by limiting the corrosivity of the product or by the appropriate choice of material for the equipment. In practice, either the level of certain impurities is specified or the degree of corrosion of a specific metal which is allowable under standardized test conditions is specified. In either event, it is necessary to determine the susceptibility of a metal to corrosion under the expected conditions of use. A number of methods have evolved to this end. The copper strip corrosion tests approved by the ASTM such as ASTM D 130 and D 1838-84 are examples. Copper is chosen in these tests because it is the most corrosion sensitive metal encountered by most petroleum products. In many cases sulfur containing species such as hydrogen sulfide, mercaptans, organic sulfides, carbonyl sulfide and even elemental sulfur are the corrosive agents of prime concern in petroleum refinery streams.
The Copper Strip Corrosion Test for LPG Gases (ASTM D 1838) together with similar methods, is a traditional type test, in which a metal token is exposed to the corrosive environment under specified conditions for a given period of time after which the token is inspected visually for any tarnish and compared to a standard color chart available from the ASTM to make a determination of the corrosivity of the selected environment. The test is clearly laborious, time consuming, imprecise and dependent upon the visual judgment of a human operator. Because of these shortcomings, various instrumented test techniques have evolved. U.S. Pat. No. 5,332,900 (Witzke), for example, describes a method for determining corrosion of a metal test token in a corrosive petroleum stream using a reflected light technique to assign an appropriate Copper Strip Test grade and to alert refinery operators to major contaminants in the product streams. Relying as it does, however, on the formation of a corrosion film, the method still requires a period of time for the film to develop, typically a quarter hour or longer, so that prompt determination of corrosion is not possible.
U.S. Pat. No. 5,804,140 (Kishi) describes a method of determining corrosion on metal test tokens using various film thickness measurement techniques including secondary ion mass spectrometer (SIMS) and x-ray microanalyzer (XMA). Whatever the method of measurement, however, the method also requires the corrosion film on the metal to grow for a predetermined period of time before its thickness can be measured, a factor limiting the speed with which corrosion can be determined.
U.S. Pat. No. 5,155,555 (Wetegrove) describes a method for monitoring film formation on the surface of a rotating test coupon immersed in the corrosive stream. This method, however, is even less amenable to real time operation because it requires the test coupon to be removed from the stream and the thickness measured outside the test chamber by a reflected light technique.